Reacting metal oxides with a sulfate and pyrosulfate mixture

ABSTRACT

The invention relates to a process for converting oxides to sulfates. The contemplated oxides are slowly soluble or insoluble in acids and/or lyes, particularly the oxides found in nuclear fuel and/or breeder material. The oxides are mixed with a sulfate and a disulfate (pyrosulfate) and heated to between about 650* C and 800* C for a comparatively short time. The amount of disulfate should be at least stoichiometric.

United States Patent 1191 Kaiser et al.

[ Sept. 25, 1973 1 REACTING METAL OXIDES WITH A SULFATE AND PYROSULFATEMIXTURE [75] Inventors: Giinter Kaiser; Erich Zimmer, both of Julich,Germany [73] Assignee: Kernforschusanlage Julich Gesellschaft mitbeschrankter Haftung, Julich, Germany [22] Filed: Apr. 20, 1971 [21]App]. No.: 135,802

[30] Foreign Application Priority Data Apr. 21, 1970 Germany P 2O 18953.1

[52] US. Cl 423/5, 423/12, 423/57, 423/84, 423/85, 423/146, 252/301.1 R

[51] Int. Cl C0lg 56/00 [58] Field of Search 23/325, 21; 252/30l.1 R;423/5, 12,57, 84, 85, 146

[56] References Cited UNITED STATES PATENTS 3,082,068 3/1963 Schlecht etal. 23/21 X 3,399,977 9/1968 Wurm et al 23/325 Primary Examiner-LelandA. Sebastian Attorney Edwin E. Greigg 5 7 ABSTRACT 6 Claims, No DrawingsREACTING METAL OXIDES WITH A SULFATE AND PYROSULFATE MIXTURE Theinvention relates to a process for decomposing oxidic compounds whichare slowly soluble or insoluble in acids and/or lyes, more especiallymetallic compounds such as ZrO ThO or the like, mixed oxides such as(Th,U) oxide, (U,Pu) oxide or the like, and compounds having thecomposition Me'O Me" O wherein Me is a bivalent metal and Me" is atrivalent metal, such as Fe()- Cr O FeO F6203 or the like, and Me'O Me"Owherein Me is a bivalent metal and Me" is a tetravalent metal, such asCa'IiO or the like, and other oxidic compounds having two or moremetals, or in some cases also non-metals, as compound constituents.

BACKGROUND OF THE INVENTION Processes for decomposing oxidic compoundsare employed in various technical fields, for example, for processingoxide ores, for reprocessing of spent nuclear fuel, and for recovery offissile and fertile material from waste resulting from the manufactureof nuclear fuel, for example, in the manufacture of oxide particles.

A number of processes for decomposing oxidic compounds are at presentknown. Thus, a method for decomposing naturally occuring zirconia hasalready been proposed, whereby the zirconia is first treated withboiling hydrochloric acid in order to remove impurities. The resultingreaction mixture is then fumed off with boiling sulphuric acid. By wayof example of this teaching, see the following: H. Holness, Metal lurgia(Manchester), Vol. 39, 1948, page 117; H. Trapp, Metallboerse, Vol. 21,1931, pages 15167 and 1565; F. L. Clark, British Pat. Specification No.401,756; German Pat. Specification No. 516,852; British Pat.Specification No. 291,004; and French Pat. Specification No. 654,465.

Heretofore, endeavors also have been made to decompose zirconium oxideby means of alkali disulphate or alkali hydrogen sulphate, which formsdisulphate (pyrosulfate) on melting. See for example: J. W. Marden, M.N. Rich, Bulletin Bureau of Mines No. 186 (1921) 8, 48; L. Weiss, R.Lehmann, Z. Anorg. Chem. 65, (1910) 178/227, 190; J. H. De Boer, A. E.Van Arkel, Z. Anorg. Chem.l4l, (1924) 284/8; J. H. De Boer, Ind. Eng.Chem. 19 (1927) 1256/9. However, the prior art discloses that the amountof melt required was very high. It amounted to five to 20 times thequantity of weight of ore. In addition, the reaction times were verylong.

It is also known that thorium oxide can be converted into water-solublethorium sulphate by reaction with hydrogen sulphate. For example, see T.sollmann, E. D. Brown, Am. J. Physiol. 18, 1907, pages 426/56.

Another method of decomposing oxidic compounds, which is employed fornuclear fuel particles, consisting of mixed oxides of thorium anduranium, resides in employing for the dissolution concentrated nitricacid, to which fluoride ions are added as catalyst. However, even thisstep involves considerable expenditure of time, especially when the fuelmaterial is present in the form of highly sintered mixed oxideparticles. Generally, about 12 hours, and in special cases even up to 75hours, were necessary. (See the report of Oak Ridge National LaboratoryTM-867). In order to obviate this disadvantage, alkali disulphate wasemployed as decomposing agent in accordance with an unpublishedproposal. In this case, the ratio by weight of disulphate to the fuelmaterial to be decomposed is about 5:1. A considerable reduction of thereaction time is thereby achieved. However, in order to decompose thereaction material quantitatively, it was necessary for the oxidematerial to be divided into small quantities which were added inportions to the molten disulphate.

Consequently, all the known processes have the disadvantage that more orless long reaction times are required. A further disadvantage resides inthat, in the hitherto known processes for the decomposition of oxidiccompounds in which the decomposition takes place by means of disulphate,the quantity by weight of disulphate employed is disproportionately highin relation to the quantity by weight of material to be decomposed. Inaddition, pyrolysis of disulphate will cause considerable losses of thisreagent, because S0 escapes from the reaction vessel without having beenutilized. If hydrogen sulphate is employed for the decomposition, thedisulphate must first be formed in the reaction vessel from the startingproduct present as hydrogen sulphate by splitting-off of water beforethe decomposition reaction can commence.

THE INVENTION The primary object of the invention is therefore toprovide a process for decomposing oxidic compounds, more especially fuelmaterials for thorium-uranium and uranium-plutonium reactors, which isconsiderably more economical than the hitherto known processes in regardto the time expended in carrying it out and reagent losses which cannotbe avoided.

To solve this problem, the decomposition is carried out by a process ofthe aforesaid kind at a temperature between about 650 C and 800 C bymeans of a mixture consisting of disulphate and sulphate, the quantityof disulphate having to correspond at least to the stoichiometricquantity, in order to convert the oxides into sulphates. It is verydesirable for the decomposition to be carried out by means of an excessof disulphate, but this excess should preferably not exceed 50 percentof the stoichiometric quantity.

It is advantageous to employ alkali metal and/or ammonium disulphatesand sulphates known per se. In addition, it is advantageous to close offthe reaction vessel by means of a wash bottle in order to obtain in thevessel a pressure of reaction gases in excess of atmospheric pressure.

To carry out the method of this invention as economically as possible,the amount of sulphate to be charged to the reaction mixture isdetermined in each individual case by first converting a sample ofoxides to sulphates by pure disulphate. Towards the end of theconversion process the ratio between metal irons and sulphate ions isdetermined by, for instance, dissolving the melt or a portion thereof inwater and titrating the obtained acid. It can then by simple calculationbe ascertained how much sulphate has been formed by pyrolysis of thedisulphate. Thus, the most economical amount of sulphate to be used inthe sulphate-disulphate mixture can be ascertained.

By application of the process according to the invention, it is ensuredthat metallic oxides, metallic mixed oxides and compounds of the generalcomposition Me'O Me" O in which Me is a bivalent metal and Me" is atrivalent metal, as well as compounds of the composition Me'O Me"() inwhich Me is a bivalent metal and Me" is a tetravalent metal, and similaroxidic compounds comprising two or more metals, and optionally alsonon-metals, as compound constituents, are quantitatively decomposed in ashort time. A further great advantage of the process according to theinvention resides in that the quantity of disulphate required to carryout the process is considerably smaller than in the case of the hithertoknown processes in which only pure disulphate or hydrogen sulphate hasbeen employed. A further advantage resides in that the sulphate employedas addition in the process according to the invention can be recoveredfrom the aqueous solution of the melt, for example, by evaporation ofthis solution after separation of the element to be reclaimed from thedecomposed compound. Finally, the difficulties existing in the hithertoknown processes in regard to the removal of waste gases formed duringthe decomposition reaction, and the resultant corrosion effects areconsiderably reduced by the process according to the invention.

EXAMPLE 1 1 kg zirconia, which was first substantially freed fromimpurities by boiling with concentrated hydrochloric acid, was heated toabout 750 C together with 6 kg of potassium disulphate and 1.4 kg ofpotassium sulphate in a quartz crucible. In order to reduce the escapeof gaseous sulphur trioxide the reaction vessel was closed off by meansof a wash bottle filled with inert liquid. As inert liquid a hydrocarbonmixture having a boiling point about 200 C was employed. About 30minutes after the reaction temperature had been reached, thedecomposition was complete. Thereafter, the melt was dissolved in waterand the zirconium was precipitated in a manner known per se as ahydrated oxide by the addition of an aqueous ammonia solution. A form ofzirconium oxide which is soluble in acids was then produced from thishydrated oxide by drying.

EXAMPLE 2 For the decomposition of thorium-uranium mixed oxides, 1 kg ofthorium-uranium mixed oxide nuclear fuel particles was firstmechanically or chemically freed from structural and coating materialsby one of the known methods. Thereafter, the nuclear fuel was introducedinto a quartz reaction vessel together with 3 kg of potassium disulphateand 1.5 kg of K 80 and heated in an oven to a temperature substantiallybetween 750 and 800C. Instead of a quartz reaction vessel, it is ofcourse possible to employ a platinum reaction vessel. In order to reducethe escape of gaseous sulphur trioxide the reaction vessel was closedoff by means of a wash bottle filled with inert liquid. As inert liquida hydrocarbon mixture having a boiling point above 200 C was employed.It was found that the reaction was complete about 30 minutes after thereaction temperature had been reached. Thereafter, the liquid melt wasdrained out of the reaction vessel in conventional manner by opening thefreeze valve employed and dissolved in water.

Of course, it is alternatively possible to employ a siphon system or anyother known means. The further processing of the reaction material maytake place, for example, by first so concentrating the solution that thethorium present in solution precipitates as a sparingly soluble doublesalt of thorium sulphate and potassium sulphate, whereafter the uraniumis extracted from the residual solution with tributyl phosphate in afurther process step. That which is claimed is:

1. A process for decomposing oxidic compounds selected from the groupconsisting of ZrO ThO mixed thorium-uranium oxide, mixeduranium-plutonium oxide, l-"eO'Cr O FeO'Fe O and CaO'TiO wherein thedecomposition is carried out at a temperature between about 650C and800C by means of a mixture consisting of a pyrosulfate selected from thegroup consisting of alkali metal and ammonium pyrosulfates and a sulfateselected from the group consisting of alkali metal and ammonium sulfatesand the quantity of pyrosulfate corresponds at least to thestoichiometric quantity, in order to convert the oxides into' sulfates.

2. The process of claim 1, wherein the decomposition takes place bymeans of an excess of pyrosulfate.

3. The process of claim 1, wherein the excess of pyrosulfate is up to 50percent of the stoichiometric quantity.

4. The process of claim 1, wherein the escape of reaction gases isreduced.

5. The process of claim 1, wherein the added quantity of sulfate isdetermined by first decomposing the material which is to be decomposedwith pure pyrosulfate and ascertaining in a known manner towards the endof the decomposition the ratio of metal ions to sulfate ions.

6. The process of claim 1, wherein an excess of sulfur trioxide ismaintained during the reaction.

2. The process of claim 1, wherein the decomposition takes place bymeans of an excess of pyrosulfate.
 3. The process of claim 1, whereinthe excess of pyrosulfate is up to 50 percent of the stoichiometricquantity.
 4. The process of claim 1, wherein the escape of reactiongases is reduced.
 5. The process of claim 1, wherein the added quantityof sulfate is determined by first decomposing the material which is tobe decomposed with pure pyrosulfate and ascertaining in a known mannertowards the end of the decomposition the ratio of metal ions to sulfateions.
 6. The process of claim 1, wherein an excess of sulfur trioxide ismaintained during the reaction.